Which antivirus for Win XP?

Much to my wife's delight, we've been given a pretty good second-hand PC walking Windows XP (she never got on with our now-dead Mandriva box, it's a long story). This means that not only do I own a Windows box for the first time in my life, I also get to administer one for the first time in over ten years. Wish me luck!

I'm planning a full format-reinstall cycle to get it out of Walking Through Treacle Mode, as well as adding a few Linux partitions etc, and then comes the big question: what kind of antivirus would be best for a home setup?

The choice is between Sophos (my employer offers me a free home licence) and any of the free offerings such as AVG, but I can't find any recent reviews of Sophos. Does anyone here have any experience with it?

For pure detection rates in free Windows AV, most seem to agree Avira Personal Edition is best.
http://www.virusbtn.com/news/2009/03_23.xml (for example)
even out performing commercial AV packages... though these results are bound to depend on what they throw at them, so should probably be taken with a pinch of salt.

I've had Windows XP running in a VM and using AVG Free for quite some time. It doesn't get a lot of use, so it's been very little trouble.
Assuming your wife is a sensible surfer, then I would think AVG Free would be perfectly adequate, but if any kids come near the computer, then I'd look for something where you get proper support and add an anti-spyware app and perhaps an interactive firewall.

Thanks for the replies. She is a very careful surfer but that doesn't stop me being paranoid, especially given the amount of time the box tends to spend online.

Sophos would get its updates directly from the mothership, so that's my preferred option at the moment. Avira looks good but I'm slightly put off by the lack of behavioural detection in the free version. I might see about using AVG or Avira for the odd offline scan to cover the gaps (did I mention I was paranoid? )

I used to be a big fan of the free AVG and installed it on all my Windows PCs after losing patience with Norton when it became steadily more and more bloated with each new version.

Unfortunately AVG seems to be following the same route and I have now removed version 2011 from my Windows machines after this latest edition recently started repeatedly popping up with various error messages, such as “Unspecified error occurred in AVG. Would you like to send diagnostic data to the AVG Technical Support department for further analysis?” I am not very keen on an AV program which reports "unspecified errors"!

I now simply rely (foolishly or otherwise) on the free Microsoft Security Essentials.

re: poll; none of the above. NOD32 from ESET.com is what you need. The only bit of Windows software I have ever paid for. And not because I use pirated software. As a former Software Engineer, I am very anti-piracy, and the same with music {I'm a Digital Musician now}. I refuse to download pirated music. I like to pay the ''artist'' responsible for the good work they have done. The only Windows Software I have bought is NOD32, Windows 95, Windows 98, Windows XP Pro (I always used to build my own hardware using the best hardware I could afford that was available), and then Windows 7 Home Premium 64 bit, which is actually quite good, but that came ''free'' with my Laptop.....
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Benjamin Kenobi.
www.newmatrixism.com